After decades of marine fish culture research, the production of most marine fish species still relies on live-food technology to sustain finfish larvae through the first weeks of life. Zooplankton—microscopic and semi-microscopic invertebrate animals existing in fresh, brackish and seawater—are frequently used as such live food. In addition to rotifers [Phylum Rotifera] several other types of zooplankton, particularly those of the free swimming genus, may be used to sustain finfish larvae through the first weeks of life, such as Sub-Phylum Crustacea, order Cladoceran (e.g., Moina 
sp., Daphnia sp.), Sub-Phylum Crustacea, sub-class Copepoda (e.g., Cyclops), and Brine shrimp (Anemia sp.). Efficient production of zooplankton, however, limits the commercialization of marine finfish.
Unlike freshwater finfish, the larval stage of many marine species of commercial interest are small and require small foods with sustained production to rear the larvae through the critical early stage. Thus, because marine finfish larvae require live feeds from the first week to about one month of their existence, there is a current need for very small live food in marine fish larviculture.
The zooplankton B. rotundiformis is an example of one such live food, but in order to serve as a viable option for many marine finfish larvae, the rotifers must be of the S or SS-types, that is, with Loric lengths from 160 micrometers down to less than 100 micrometers. The inability to supply microalgal/zooplankton feed in a consistent, cost effective manner continues to be a limitation to the expansion of the marine aquaculture industry.
The primary technologies available in the aquaculture industry for the culturing of zooplankton rely on batch processing methods. Batch process cultures are harvested only once before beginning the culture cycle again, thus necessitating redundant systems to ensure a continuous zooplankton supply and thereby increasing the equipment footprint and labor needs. Further, because batch cultures are grown to maximum standing crop prior to harvest, stability of the culture becomes an issue, often resulting in high frequency of culture collapse. Finally, the consistency and quality of batch processed zooplankton cultures can be highly variable. While semi-continuous culture systems are an improvement over batch culture systems, they remain primarily manually operated and thus labor-intensive. Therefore, an automated, continuous zooplankton culture system will fill a need in the aquaculture industry.
In order for such an automated, continuous culture system to function with reasonable efficiency, it is necessary to determine, in-process, the concentration or density of zooplankton present in the system. In order to properly control the growth of the organism in a computer-supervised system, for example, information on the zooplankton's density in the culture water must be passed to a control unit. In the past, this density has been determined by such means as a turbidity measurement, machine vision methods, hemocytometry, or manual counts under a microscope. These methods are costly to implement, require frequent instrument calibration and/or cleaning, may require large amounts of human effort/time, and are impractical for an application wherein this information is needed on a real-time basis.
Thus, for a device and/or method for determining the concentration of suspended particles, such as zooplankton, to be both economical and practical for a process control system, it must circumvent the above factors to in order to allow the control system to automate the process to its fullest possible potential with minimal control required on the process water itself and minimal maintenance time by human operators.